In recent years, development of hybrid cars and electric vehicles has been advanced rapidly and accompanied by this, a braking method of a vehicle has been changed from a conventional mechanical hydraulic control method to an electric hydraulic control method, so that a variety of control methods have been proposed.
Generally, the battery is used as a power supply in order to control hydraulic pressure of a vehicle electrically. In that case, if no other thing but the battery is provided, hydraulic control cannot be performed when supply of electricity is interrupted for some reason, so that braking of the vehicle is disabled.
Therefore, there has been proposed a charging apparatus equipped with a so-called auxiliary power supply in order to cope with an emergency by equipping a large-capacity capacitor or the like as the auxiliary power supply in addition to the battery.
It is very important for the auxiliary power supply for use in braking the vehicle to supply electricity securely in an emergency and the capacitor needs to be charged rapidly from a capacitor discharging state at the time of engine start to a predetermined voltage.
As a background art document relating to this application, for example, Unexamined Japanese Patent Publication No. 5-116571 is known concerning a charging apparatus for battery auxiliary capacitor for engine start.
In the conventional charging apparatus for braking of the vehicle, more specifically, a capacitor having a capacitance of several tens Farads after the engine is started is required to be charged rapidly up to a predetermined voltage in a relatively short time of about 100 seconds.
FIG. 13 shows an example of the conventional charging apparatus which charges with a constant current. This circuit operation is as follows. That is, charging current I is supplied from a constant voltage source V to capacitor 2 having a capacitance of several tens Farads through charging element 1 attached to a radiator plate (not shown). Charging current I is detected by current detecting portion 3 and converted to voltage V3, and then inputted to first input terminal 4a of constant-current-control-circuit 4. Reference voltage 5 is supplied to second input terminal 4b of constant-current-control-circuit 4. A voltage corresponding to a difference between voltage V3 detected by current detecting portion 3 and reference voltage 5 is taken out to output terminal 4c of constant-current-control-circuit 4. That is, constant-current-control-circuit 4 amplifies a difference between voltages inputted to first input terminal 4a and second input terminal 4b. Voltage V4 taken out from output terminal 4c of constant-current-control-circuit 4 is fed back to a control terminal side of charging element 1 through resistor 6. Consequently, charging current I flowing to charging element 1 is controlled to a constant level so that capacitor 2 is charged up to a substantially equal voltage to constant voltage source V.
FIG. 14A, FIG. 14B and FIG. 14C show change over time of each characteristic of the conventional charging apparatus shown in FIG. 13. The abscissa axis of FIGS. 14A to 14C shows charging time t. A charge start time is indicated with t0 and a charge completion time is indicated with t2 (≈100 seconds). The ordinate axis of FIGS. 14A to 14C shows various characteristics. The ordinate axis of each of FIGS. 14A, 14B and 14C indicates change over time in charging voltage VC and charging current I of capacitor 2, loss power W consumed by charging element 1, surface temperature TH of charging element 1 and internal temperature Tjc of charging element 1.
When charging is started at charge start time t0 in FIG. 14A, charging current I of a specified value flows to capacitor 2 as shown in FIG. 14A because the charging apparatus shown in FIG. 13 is a type which executes constant current control. Consequently, charging voltage VC of capacitor 2 rises with time and becomes substantially equal to a voltage of constant voltage source V at charge completion time t2.
FIG. 14B shows change over time in loss power W consumed by charging element 1 in charging process. That is, because no charging voltage VC is applied to capacitor 2 at an initial stage of charging, a voltage of constant voltage source V is applied to charging element 1. After that, as the progress of charging, charging voltage VC of capacitor 2 rises and a voltage applied to charging element 1 lowers. Thus, loss power W indicates a maximum value at charge start time t0 as shown in FIG. 14B and after that, it lowers as the charging progresses.
FIG. 14C shows change in temperature of charging element 1 during charging. In charging element 1 originally at room temperature TO, internal temperature Tjc thereof rises due to generation of heat by loss power W. According to this change, surface temperature TH of charging element 1 rises.
However, as evident from FIG. 14B, loss power W lowers gradually as a charging time passes. Thus, as shown in FIG. 14C, internal temperature Tjc of charging element 1 indicates a maximum value Tjcmax at time t1 and internal temperature Tjc thereafter lowers with passage of time t. According to this change, surface temperature TH of charging element 1 shows similar temperature change.
A problem caused by such a temperature change is that the inside of charging element 1 is placed under a high temperature condition by loss power W. In a word, every time the vehicle is started, the inside of charging element 1 receives a thermal shock.